How do you land a spaceship on a comet?

Breaking Records, Taking Measurements

When the Philae lander touched down, it became the first craft ever to make a controlled landing on a comet nucleus, but this is far from the only record it will set. Remarkably, it will be the first craft to venture beyond the main asteroid belt on solar power alone, despite the fact that, at 500 million miles (800 million kilometers), sunlight plunges to a scant 4 percent of Earth levels. The lander will also snap the first shots ever taken on a comet's surface, while Rosetta will become the first spacecraft to orbit a comet's nucleus, the first to fly wingman to an inbound comet and the first to witness its sun-induced changes up close [sources: ESA; ESA].

The orbiter hosts various gadgets planned to work in tandem with the lander's equipment. Ultraviolet and thermal-imaging spectrometers, along with a microwave instrument, will analyze the coma and help the lander study the comet's nucleus and coma-related outgassing. An onboard radio-wave sounder will also help Philae study the comet's internal structure. Rosetta will further analyze the coma's dust using an ion mass analyzer, a grain impact analyzer and dust accumulator, and a micro-imaging dust analysis system. Other instruments will study the comet's atmosphere, ionosphere and plasma environment, including temperature, velocity, gas flow density and magnetic field. Rosetta also sports a dual narrow-/wide-angle camera that sees in the visible, near infrared and near ultraviolet wavelengths.

The lander carries 10 experiments to observe, sample and analyze the comet's composition, supported by a drilling subsystem that can bore up to 9 inches (23 centimeters) and deliver material to onboard instruments. Among them is an alpha proton X-ray spectrometer, which distinguishes chemical elements by exposing a sample to a radioactive source and analyzing the energy spectra of bounced-back alpha particles, protons and X-rays [sources: ESA; NASA].

Philae also has a panoramic visible and infrared camera system, along with a landing imager. It will use a radio wave sounding system to map the comet's core structure and an electric sounding and acoustic monitoring system to gain a sense of the comet's mechanical and electrical characteristics. A multipurpose sensor will study surface and subsurface properties, and a magnetometer and plasma monitor will track the body's magnetic field and charged particle environment [sources: ESA].

Two gas analyzers will sort out the comet's surface makeup. One, COSAC, combines a gas chromatograph and mass spectrometer. The other, PTOLEMY, uses an ion trap mass spectrometer to analyze surface solids and atmospheric gases [sources: ESA; NASA].

It's a lot of equipment to fit into two small boxes, but decades of launching probes have taught ESA and NASA a thing or two about packing.

Author's Note: How do you land a spaceship on a comet?

I've written in previous articles about the staggering complexity of launching a spacecraft to a specific planetary site or along a particular trajectory in space. Although we know -- or, at least, study -- the orbits of many objects, planets and moons, the distances and speeds involved are, well, astronomical, to say nothing of the gravitational tugs exerted by the various masses circling the sun.

As astonishing as such achievements are, often the hardest part of a space mission is not getting there, but rather surviving the trip. We tend to take it for granted that, assuming the launch goes well and no one confuses metric with English units, the craft will function. I guarantee you the scientists and engineers who design, build, (test, test, test) and launch these craft are not so sanguine about it. As the spotty-at-best track record of early planetary probes illustrates, engineering a craft to survive the rigors of space and hibernation for months, let alone a decade (!), still rates as one of the most extraordinary feats of engineering ever attempted -- and that's before you strap your meticulously assembled collection of instruments, control systems and propulsion onto one of those controlled explosions we call rockets.

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